A tractor-trailer combination that transports cargo on roads and highways inherently has significant aerodynamic resistance due to its large shape and size. The air gap that exists between a tractor and a trailer is known to have an effect on a tractor-trailer's aerodynamic drag. Various structures or features can be added to or incorporated in the design of tractors and trailers to aid in reducing drag. Some structures and features have an association with the air gap between the tractor and the trailer that is intended to reduce turbulence at the air gap that contributes to drag.
The fifth wheel of a tractor typically has an adjustment mechanism that allows the fore-aft position of the fifth wheel to be set for a particular trailer that is to be towed. Once set, the fifth wheel is typically locked in place by some sort of mechanical locking device.